1. Field of the Invention
The present invention relates to a spectrometry technique for evaluating a piezoelectric field in an epitaxial layer without destroying the crystal.
2. Description of the Related Arts
The density of two-dimensional electron gas formed at an AlGaN/GaN interface gives the greatest influence to the performance of high electron mobility transistors (referred to as “HEMT” hereinafter) of an AlGaN/GaN type. The density of the two-dimensional electron gas is determined by a sort of internal electric fields called as a piezoelectric field induced by a lattice-mismatch strain peculiar to a semiconductor heterojunction. Therefore, the evaluation of the piezoelectric field is considered to be important upon testing the quality of epitaxial layer structures for the HEMTs.
It has been generally known that photoreflectance (PR) spectroscopy is effective for evaluating the internal electric fields in the crystal such as the above-mentioned piezoelectric field (for example, see Japanese Laid-open Patent publication No. 3-175340). In the PR spectroscopy, the reflectance change ΔR/R, which is caused by the irradiation of the pump light to the sample, is recorded as a function of photon energy with the use of the so-called probe light. In the samples with internal electric fields, the spectra of the reflectance change ΔR/R exhibits oscillatory patterns called as Franz-Keldysh oscillations (FKOs). The period of the oscillatory patterns is determined by the strength of the internal electric fields. Accordingly, the internal electric field strength can be evaluated through the analysis of the period of the Franz-Keldysh oscillations.
However, it is extremely difficult to measure and evaluate the piezoelectric field at the AlGaN/GaN interface by using the PR spectroscopy. Specifically, the analysis of PR spectra has the complicated procedure for extracting the piezoelectric field from a PR signal. The reason is as follows. In case where the PR spectroscopy is applied to the AlGaN/GaN epitaxial layer structures, the detected reflectance change ΔR/R is sum of the signals originating from the layers that produces carriers owing to the pump-light irradiation. Therefore, it includes information about the internal electric fields of various layers.
In order to solve the above-mentioned problem, pre processes, where a portion other than the vicinity of the target AlGaN/GaN interface is etched off, seem to be useful ways to perform the PR measurement. However, the etching process gives rise to the problem that the sample should be destroyed. Moreover, in the etching process, surface electric field components, which are caused by a defect introduced to the sample by the etching, may emerge. Signals associated with the surface electric fields overlap with the component originating from the piezoelectric field, thereby resulting in a more complicated signal analysis.